8.882 LHC PhysicsExperimental Methods and Measurements
Detectors: Electrons and Particle Id[Lecture 12, March 16, 2009]
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Organization
Project 1: Charged Track Multiplicity● no hand-ins as of yet
Project 2: Upsilon Cross Section● all material is on the Web● is due April 9 (in 3.3 weeks)● please, try to find a partner if you do not yet have one
CP Travel plans● at MIT in the flesh Wednesday/Friday● April 8 - May 13 as well: find alternative time for Apr 8
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Lecture Outline
Electron Identification● electromagnetic calorimetry
Particle identification systems● dE/dx in drift chamber● TOF – Time-Of-Flight detectors● RICH – Ring Imaging CHerenkov detectors● DIRC – Detection of Internal Reflected Cherenkov light
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Why Muons and Electrons? Leptons● rare in pp (<1% of the tracks), often related to very
interesting physics processes● taus special case (m = 1.777 GeV, cτ = 87.11 μm)
● decay well before they reach the silicon detector, lifetime more then a factor of five smaller then for B mesons
● can also produce hadrons in decay, more difficult to identify● always involve neutrino in decay (incomplete reconstruction)
● muons have very characteristic signature● penetrate the calorimetry, are detected in the muon chambers● leave minimally ionizing signature
● electrons have very characteristic signature● maximal ionization in tracking system● get absorbed completely in ECAL no signature in the HCAL● shower shape in ECAL is short and broad
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Why Electrons/Photons at the LHC? Physics opportunities● very low Higgs masses (below 130 GeV): H → γγ● most of other range: H → ZZ(*) → e+e-- (μ+μ--/e+e--)● Z' decaying to e+e-- final state, masses as high as possible
Requirements for the ECAL at LHC● excellent resolution over very large dynamic range
● CMS decided for crystal calorimeter● high light output
● capable of dealing with dense particle distributions● dense material to quickly contain shower● fine granularity
● capable to resist high radiation, maintaining performance● material research necessary
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Particle Identification
Electron Signature: track + all energy in ECAL● backgrounds: photons plus random track● neutral pion: decays to 2 photons (shower shape)
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Compare Electron and Muon Id
Muon identification● by definition background is quite low● few particles arrive in muon system: gold plated
Electron/Photon identification● very large number of particles● tracking essential (reject/select photons)● electromagnetic calorimetry essential (reject neutral
pions)● hadron calorimetry essential (reject other hadrons)● intrinsically more complex then muons but still very
important
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Higgs Mass Drives ECAL Design Electroweak data● Higgs < 144 GeV
Direct searches● Higgs > 114 GeV
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ECAL Performance Resolutions● ECAL benchmark: mass resolution of H → γγ process
Components: energy and angular resolutions● angular resolution can be achieved without too much
problems: more about this later● energy resolution more complex
a – stochastic proportionality factorb – constant term (calibration, non-uniformities etc.)σ
N – noise equivalent (electronics, pileup energy)
● a<10% difficult with sampling (pushes precise geometry)● a=2% with active calorimeters (requires b<0.5%, tricky)
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Higgs to gamma gamma
the narrower the mass peak the cleaner to separate
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Electromagnetic Calorimeters Sampling calorimeters● Atlas: liquid Argon, accordion geometry
Fully active calorimeters● CMS: PbWO
4 crystal calorimeter
● more sensitive to radiation● online laser monitoring system● also called homogeneous
Cloud chamber with lead absorbers
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ECAL Layout in CMS CMS choice● crystal calorimeter: PbWO
4 (compact, fast, doable)
● PbWO4 is optimal material, next page
● endcap has additional pre-shower: reject neutral pions as photon background
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Crystal Comparison
Moliere radius RM
= 0.265 X0 (Z+1.2)
● 95% transverse shower contained in 2 RM
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Energy Loss in Trackers Bethe Bloch● depends on β only● given dE/dx and
momentum p determines and β thus the mass, m
● after mass correction: universal curve
How to measure?● pulse height in tracker● lots of corrections ...
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Time-Of-Flight Detectors
Principle● arrival depends
on velocity● p + v: m
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CDF: Time-Of-Flight Detector
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Bar Arrive at the Bore
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Bar Unpacking
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Inserting the Bar
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And the Tracker still Fits
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Particle Distinction with TOF
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Cherenkov Light Particle travels through material● weak EM wave spreads: polarizing/de-polarizing effect● slower then wave speed: waves never interfere● faster then wave speed: they will interfere and create
conic light under characteristic angle: cosθC = 1/β
n
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Ring Imaging CHerenkov Detectors RHIC detectors● particle velocity from the opening angle of light cone● particle passes through proper type of material● the light cone produces a ring image● size of ring determines velocity
SuperKamiokandering LHCb Aerogel RICH
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DIRC at BaBar Experiment Detection of Internally Reflected Cherenkov light● quartz bar is used to transport light under Cherenkov
angle● light ring in water tank● size determines β● high surface quality needed● angle has to be retained
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Conclusion
Electron/Photon reconstruction crucial at LHC● very low Higgs masses drive the ECAL design● m
H below 130 GeV: H → γγ
● active calorimeter more precise then sampling type● CMS and Atlas covers full Higgs range
Particle Id (mostly pion/kaon separation)● dE/dx in tracking (solid and gaseous)● time of flight measurements● Cherenkov light cone do determine velocity
● Ring Imaging CHerenkov detectors: RICH● Detection of Internally reflected Cherenkov light: DIRC
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